The present invention generally relates to a pick-up arrangement and more particularly, to a pick-up device suitable for use in an optical disc apparatus such as a so-called compact disc player, video disc player or the like.
Commonly, an optical disc apparatus is arranged so as to accomplish recording by forming fine holes or grooves in a spiral or concentric pattern on the surface of a disc-like recording medium or by subjecting the surface of the recording medium to magnetic variations. To accomplish reproduction, a spot of light is projected onto the recorded portion of the recording medium so as to derive the reflected light or transmitted light as information reproduced from the recording medium.
In FIG. 1, there is shown a general construction of a conventional optical information reading pick-up device as incorporated in an optical disc apparatus.
In the conventional pick-up device, as shown in FIG. 1, the optical system includes a collimate lens Lm and a beam splitter Sp provided in a barrel Br so as to collect and project laser light emitted from a semi-conductor laser W onto a recording medium M through the collimate lens Lm, the beam splitter Sp and an objective lens Lo. After being subjected to modulation by the information recorded portion on the recording medium M, the light thus projected is divided by the beam splitter Sp through the objective lens Lo. One portion of the divided light is incident upon a photo-detector Pd via a detecting system convex lens Ld and a cylindrical lens Lc.
In the conventional optical information reading pick-up device having the optical system as described above, in order to correctly record the information on the recording medium or to accurately reproduce the recorded information from the recording medium, it is necessary to control the tracking process, in a manner, such that the collected light spot correctly traces the information portion (referred to as tracking) as recorded in the spiral or concentral configuration (i.e. information record track), while simultaneous controlling the focusing of the light spot.
Moreover, for actual application, since the above pick-up device is subjected to influences due to eccentricities or fluttering of the disc, and external vibrations, etc., it is necessary to provide a mechanism to derive such deviations as error signals for automatic correction. For this purpose, it has been a normal practice to accomplish the focus control and the tracking control through employment of an electromagnetic driving unit including a magnetic circuit and a coil means. For ensuring control within the high frequency region during the above tracking driving, it is desirable that the driving portion be compact in size and light in weight.
From the above point of view, it has also been proposed in a conventional arrangement that only the objective lens Lo be separated from the barrel for driving, However, in such a construction, deviation between an optical axis of the pick-up and that of the objective lens is realized, thus resulting in the deterioration of the quality of the pick-up control signal causing some countermeasures to be necessary. Furthermore, in order to avoid the deterioration of the recording signal and reproducing signal with respect to the disc, a driving unit having a sufficiently high accuracy is necessary so as to prevent inclination of the lens optical axis from taking place during driving of the objective lens.
Meanwhile, in the pick-up device, as shown in FIG. 1, when the whole pick-up assembly is driven, cross-talk between the tracking control and the focus control tends to take place since the barrel Br is not formed with axial symmetry. More specifically, on the assumption that the pick-up is driven in a direction at right angles with respect to the recorded surface of an information carrier or recording medium having a disk-like shape, a phenomenon where the optical axis deviates from the direction normal to the recorded surface, is difficult to avoid when a pick-up device is used which does not have axial symmetry.
Additionally, in the conventional pick-up device as shown in FIG. 1, the astigmatism producing optical system including the convex lens Ld and the cylindrical lens Lc is necessary to obtain the focusing error signal for accomplishing the focus control. However, such an optical system becomes an obstruction in realizing the reduction of size, weight and cost for the pick-up device.